As a conventional technology, for example, it is proposed in Japanese Patent Publication No. 2871891 that a ballast stores or estimates a lamp voltage when a lamp is lighted at rating, and in response to a value of the stored or estimated lamp voltage, controls lamp power at the time when the lamp is actuated next time. However, in this document, mainly described are power control for one and the same lamp, which corresponds to aging thereof, and control for the power when the lamp is in a restarting state (hot restart), and accordingly, in accordance with this technology, it has been impossible to control the power in order to drive lamps different in rating power.
In Japanese Patent Laid-Open Publication No. 2005-19137, it is described that a data table describing output power characteristics of lamp voltage-lamp power of a high intensity discharge lamp is provided, constant power control is performed in a range where the lamp voltage is from a rating lamp voltage V1 to a lamp voltage V2 (>V1) in the end of a lifetime of the lamp, and when the lamp voltage exceeds V2, control is performed so as to increase a lamp current more than in the constant power control. A technology described in this document aims to ensure necessary illuminance even at the time of driving the lamp in which the lifetime nearly reaches the end. Accordingly, in accordance with this technology, it has been impossible to perform power control corresponding to a difference in characteristics among lamps, each of which is at the beginning of a lifetime thereof.
The present invention has been made in consideration for such points as described above. It is an object of the present invention to optimally control power outputted from a high intensity discharge lamp lighting device to loads connected thereto in response to characteristics different for each of discharge lamps while coping with a difference among the loads, such as a difference in gas components contained in arc tubes, and a difference in shape of the arc tubes.